Why focus on speed?

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Why focus on speed?

1Why focus on speed? 1.1 Road traffic crashes and injury involving speed. . . . 3 1.1.1 Speed, energy transfer and injury . . . . . . . . . . . . . . . . . . . . 3 1.1.2 How does speed affect road traffic collisions and injury? . . . 6 1.1.3 What factors contribute to speeding?. . . . . . . . . . . . . . . . 10

1.2 What is speed management? . . . . . . . . . . . . . . . . . . . . 11 1.2.1 Aims of speed management . . . . . . . . . . . . . . . . . . . . . . . 11 1.2.2 Setting speed limits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.2.3 Safe systems and the role of speed. . . . . . . . . . . . . . . . . . 14 1.2.4 Benefits of speed management . . . . . . . . . . . . . . . . . . . . 17

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1 | Why focus on speed?

Speed management: a road safety manual

This module provides background information on why speed is a risk factor in road traffic crashes and injury, and the importance of tackling it with a range of different measures. In order to successfully promote, design and implement a speed management programme, it is important to understand the role of speed in road traffic crashes, and the relationship between speed and the severity of those crashes. Such information is important in persuading political leaders, stakeholders and the public to support a speed management programme. The module is divided into two sections: 1.1 Road traffic crashes and injury involving speed: This section describes the nature of crashes and speed related injury. It demonstrates how unsafe motor vehicle speeds can increase both the risk of a collision and the severity of injury to crash victims. The impact of speed on vulnerable road users, particularly in developing countries, is described. Reasons why people drive at unsafe speeds are discussed. 1.2 What is speed management? This section discusses the definition of speed management ? an active approach that requires (or persuades) drivers to adopt speeds that offer mobility without compromising safety. The Safe-system approach aims to achieve a road transport system that anticipates and allows for human error, while minimising the risk of death or serious injury. Benefits of speed management are discussed, and the impact of even small reductions in speed on safety is described. This manual does not suggest that higher speeds cannot be beneficial. Shorter journey times can provide economic benefits and increased mobility. But policy-makers must trade these benefits against the increased costs of death and injury that might occur. This manual presents the road safety case for speed management, and offers practical advice on how to manage speeds in order to deliver road safety benefits. This manual does not consider speed related noise or air pollution, or energy consumption. Although these issues are important, they are beyond the scope of this manual.

1.1 Road traffic crashes and injury involving speed

1.1.1 Speed, energy transfer and injury Speed has been identified as a key risk factor in road traffic injuries, influencing both the risk of road traffic crashes and the severity of the injuries that result from them (1, 2, 3). Higher speeds lead to a greater risk of a crash and a greater probability of serious injury if one occurs. This is because, as speed increases, so does the distance travelled during the driver's reaction time and the distance needed to stop. Also, at speed, the effects of drivers' errors are magnified. In a crash, the higher the speed

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Module 1: Why focus on speed?

the greater the amount of mechanical (kinetic) energy that must be absorbed by the impact. Hence, there is more likelihood of serious injury. According to research (1, 2), harmful injury is the result of `energy interchange'. During a collision, injury results from the transfer of energy to the human body in amounts and at rates that damage cellular structure, tissues, blood vessels and other bodily structures. This includes kinetic energy, for example when a motor vehicle user's head strikes the windshield during a crash. Of the various forms of energy ? kinetic, thermal, chemical, electrical and radiation ? kinetic energy transfer is the biggest contributor to injury. It is useful for road traffic injury prevention researchers and practitioners to understand the biomechanics of kinetic energy injuries. This will help them develop measures that will limit the generation, distribution, transfer and effect of this energy during a road traffic collision (2). Regardless of whether the kinetic energy is generated by a motor vehicle crash, a gunshot or a fall, the force to which human tissue is subjected on impact is the product of the mass and velocity involved. The kinetic energy to be absorbed equals one half of mass multiplied by the square of velocity ? illustrating that the effect of velocity is greatly enhanced as velocity increases. The level of damage to the body will depend on the shape and rigidity of the colliding surface or object, but velocity usually plays the most critical role (4). In a crash, it is physically impossible for any occupant to securely hold an unrestrained object, such as a child. In a collision of just 50 km/h, the child's weight will effectively increase by 20 times and a 5 kg baby will appear to weigh 100 kg within a split second. Source: (5).

Vulnerable road users such as pedestrians, cyclists, moped riders and motorcyclists have a high risk of severe or fatal injury when motor vehicles collide with them. This is because they are often completely unprotected or, in the case of a motorcyclist, have very limited protection. The probability that a pedestrian will be killed if hit by a motor vehicle increases dramatically with speed. In Figure 1.1 the probability of a fatal injury for a pedestrian colliding with a vehicle is illustrated. The research indicates that while most vulnerable (unprotected) road users survive if hit by a car travelling 30 km/h, the majority are killed if hit by a car travelling at 50 km/h (6).

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Speed management: a road safety manual

? HJ Sommer, GTZ, 2003

1 | Why focus on speed?

Figure 1.1 Probability of fatal injury for a pedestrian colliding with a vehicle

100% 80% 60% 40% 20% 0%

0

10

20

30

40

50

60

70

Impact speed (km/h)

Source: (6)

In the majority of serious and fatal crashes, injuries are caused because loads and accelerations ? exceeding those that the body can tolerate ? are applied by some part of the car (7). The human tolerance to injury by a car will be exceeded if the vehicle is travelling at more than 30 km/h. Pedestrians, as illustrated above, incur a risk of about 80% of being killed at a collision speed of 50 km/h. For car occupants, wearing seat-belts and using well-designed cars generally can provide protection to a maximum of 70 km/h in frontal impacts, and 50 km/h in most side impacts (8). Higher speeds could be tolerated if the interface between the road infrastructure and vehicle were well designed and crash protective ? for example, by the provision of crash cushions on sharp ends of roadside barriers. However, most road systems allow much higher speeds without the protective barriers between vehicles and roadside objects.

The unpredictable nature of human behaviour in a complex traffic environment means it is unrealistic to expect that all crashes can be prevented. But if greater attention were given to the tolerance of the human body to injury when designing the transport system, there could be substantial benefits when crashes do occur, meaning they might not lead to serious injury or death. Most traffic systems, however, are not designed on the basis of human tolerance. Separating cars and pedestrians by providing footways is very often not done. Speed limits of 30 km/h in shared-space residential areas are often not implemented. Historically, car and bus fronts have not been designed to provide protection for pedestrians against injury at collision speeds of 30 km/h or more.

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Module 1: Why focus on speed?

1.1.2 How does speed affect road traffic collisions and injury? Most road safety experts agree that the single most important contributor to road fatalities around the world is poor speed selection, commonly interpreted as the use of inappropriate vehicle speeds, or `speeding'.

Definition of speeding

It is useful to establish a working definition of `speeding' for the purposes of police assessment of the role of speed in a crash. The definition for general application in this manual is drawn from OECD, ECMT (2006) which is: "Speeding encompasses excessive speed (driving above the speed limit) or inappropriate speed (driving too fast for the conditions, but within the limits)."

Source: (6)

Higher speeds increase the risk of a crash for a number of reasons. It is more likely that a driver will lose control of the vehicle, fail to anticipate oncoming hazards in good time and also cause other road users to misjudge the speed of the vehicle. It is clear that the distance travelled in a given time ? and so the distance travelled as a driver or rider reacts to an unsafe situation on the road ahead ? is greater for travel at a higher speed. In addition the stopping distance for a vehicle, after a driver reacts and brakes, will be longer at a greater travel speed. Studies have shown that reaction time can be a little as one second, but in one trial (9) it was found that most response times were between 1.5 and 4 seconds. The consequences of such factors are illustrated in Figure 1.2. The figure shows driver reaction distances and braking distances in metres to illustrate what can happen if a child runs out into the road at a point about 13 metres in front of a car. If the car is travelling at 30 km/h it can just stop before hitting the child, but if the speed of the car is 50 km/h, the distance covered in the driver's reaction time (14 metres) is more than the distance to the child. Consequently, the child will be hit by the car travelling at 50 km/h and the chances of it surviving are small. Excessive and inappropriate speed is the biggest road safety problem in many countries (6). While identifying contributory factors in traffic crashes can be somewhat subjective, there are surveys (10) and studies (11) that suggest that as much as one-third of collisions resulting in a fatality involve an element of excess speed. Speed is an aggravating factor in all crashes.

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Speed management: a road safety manual

? HJ Sommer, GTZ, 2003

1 | Why focus on speed?

Figure 1.2 Illustration of the stopping distance in an emergency braking

0 30 km/h

Distance (metres)

5 10 15 20 25 30 35 40 45 50 55 60 65

40 km/h

50 km/h

Reaction

Braking

60 km/h

70 km/h

80 km/h

Source: (6) adapted from the Australian Transport Safety Bureau

CASE STUDY: Speed related crashes, New Zealand

Frith et al (11) attributed 31% of all fatalities and 17% of all serious injuries in New Zealand to speeding in the year 2002, based on police judgements. They further stated that these levels were likely to underestimate the full impact of speed on crashes and crash severity, given that speed contributes to the severity of crash outcomes regardless of the cause.

They indicated that as a broad estimate, if the average speed on New Zealand's rural roads were reduced by just 4 km/h, the total number of road crash deaths would decrease by about 15% and the total number injured by about 8% ? meaning that about 45 deaths and 480 reported injuries would be avoided (the difference between the fatal and serious injury proportions reflects the greater impact of any speed reduction on the most severe injuries).

Small increases in speed result in large increases in crash risk

Studies provide direct evidence that speeds just 5 km/h above average in 60 km/h urban areas, and 10 km/h above average in rural areas, are sufficient to double the risk of a casualty crash ? roughly equivalent to the increase in risk associated with a blood alcohol concentration of 0.05 g/100 ml (the blood alcohol limit for driving in many countries). The evidence also indicates that `moderate speeding' (within 10 or 15 km/h of the posted limit), makes a large contribution to serious road crashes ? comparable to the contribution of more extreme speeds ? because it is so common.

Source: (12, 13)

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Module 1: Why focus on speed?

The problem of speeding has increased over the years since the maximum speed that new cars are capable of is, in many cases, double the existing speed limit in rural areas. Many modern cars now are easily capable of speeding, which was typically not the case when speed limits were first introduced. It is therefore more of a challenge to convince drivers to drive within posted speed limits.

As Figure 1.3 demonstrates, the development of engine technologies over the past 40 years has resulted in most cars having a top speed well in excess of maximum speed limits (6). This presents challenges in managing travel speeds to within limits for both high-level and low-level speeders.

Figure 1.3 Percentage of vehicles sold in France capable of travelling more than 150 km/h

% of vehicles ? HJ Sommer, GTZ, 2003

100 90 80 70 60 50 40 30 20 10 0 1967

1972

1980 Year

1987

2006

CASE STUDY: Effect of changes in speed limits

A review of the studies on speed limit changes from several countries (South Africa, Belgium, Finland, France, UK, Germany, USA and New Zealand) where a speed limit was reduced or a new limit was introduced found a reduction in road crashes ranging from 8% to 40% (14).

Research in America (15) examined the effect of changes in speed limits on deaths on rural interstate highways. Road crash deaths in the groups of states that raised their speed limits from 65 to 70?75 mph rose by 38% and 35% respectively, relative to fatality levels in the states that did not change their speed limits.

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